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I'm thinking of buying one of the cheap continuous lighting kits, specifically with softboxes or shoot-through umbrellas.

I'm experienced with strobes, but I only used to use the professional grade ones in rented studios and I can never afford to buy those things for myself (and I wouldn't want to buy cheap ones with 5-7secs recycle times). I also thought I can use the continuous lights for video!

My question is, how much power do I need, to get decent (portrait) exposure with my camera set to say, f6.3, 1/80, ISO400 ? Or to be ambitious, f8, 1/125, ISO200?

Would a 5x45w CFL head, through an umbrella/softbox cut it?

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relevant: photo.stackexchange.com/q/34225/9161 –  Bart Arondson Jun 13 at 14:30
    
It depends entirely on distance, the inverse square law can be your friend, get the light 4 x closer and it will be 16 x brighter! –  Matt Grum Jun 14 at 0:31
    
OK, let's say a 20x28" softbox 3ft away from my subject's face. My camera is set to f6.3, 1/80, ISO400? –  Rollo R Jun 14 at 5:25
    
Also relevant is: photo.stackexchange.com/questions/36158/… –  James Snell Jun 14 at 12:13

1 Answer 1

up vote 1 down vote accepted

You'll be able to make a good guess by testing on a speedlight or a studio strobe. Work in a darkened room so that the entire exposure comes from your lights rather than the ambient, and see what power setting you need for getting a good exposure.

Flashes dump their output in around 1/1,000 of a second. Let's say your 100W/s speedlight gives good exposure on full power, this means you're using the equivalent of a 100,000W bulb for just 1/1,000 of a second, so to get an exposure with equivalent f-stop and ISO in 1/80 (.0125s) you'd need to use 100,000 x .0125 = 1.25KW of lighting.

You'd have to research exactly what power rating your flash was and how quickly it dissipates 100% of its power, but I think 5x45W is not likely to be powerful enough to provide the f/6.3 1/80 ISO400 results you want.

Hell, an even easier way to work out how close 5x45W will get you is to stick a single 100W bulb in a soft box and change your exposure by 1.5 stops - f/6.3 1/80 ISO2,000 and see if you are even close to where you want to be.

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Good answer with both mathematical and practical approach. It turns out, a better bang-for-the-buck would be some cheap speedlights (like a Yongnuo 560) for photography and 2 single 45w bulbs for videos, which I can mount on the same stand+umbrella. –  Rollo R Jun 23 at 15:16
    
Glad to have helped. I think the Yongnuo approach is a good one - the 560s even have built in radio triggers so you don't have to mess about with cables. My experience tends to be that continuous lighting isn't great for stills photography unless you're happy working with a wide aperture/high ISO or you can afford (and are strong enough to carry) very high output lights. –  user29742 Jun 23 at 15:29

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